1. Field of the Invention
The present invention relates to a fixed platen used in an injection molding machine.
2. Description of the Related Art
A mold clamping mechanism of an injection molding machine is equipped with a fixed platen fixed to a base frame, and a movable platen disposed to be movable with respect to the fixed platen, while maintaining a posture that is parallel to the fixed platen. Furthermore, a fixed-side mold is mounted to the fixed platen, and a movable-side mold is mounted to the movable platen. These molds are opened and closed, and clamped by moving the movable platen forward and backward with respect to the fixed platen.
The mold clamping mechanism is required to reliably clamp the fixed platen and the movable platen against the pressure of resin to be injected into the mold. Moreover, the fixed platen is supported by tie bars that are generally disposed at four corners thereof.
Here, an example of a conventional mold clamping mechanism will be described with reference to FIG. 9A (front view) and FIG. 9B (side view).
Since the mold clamping mechanism receives a reaction force of the mold clamping force from the molds (a fixed-side mold 4 and a movable-side mold 5) mounted on central portions of mold mounting surfaces when the molds are clamped, the mold mounting surfaces receive the pressure in a manner such that they are deformed into a bowl shape. Therefore, force escapes from the central portion of the mold and a contact surface pressure is lowered, and thus, there is concern that burr occurs in a molded article. Furthermore, on the contrary, when the excessive pressure locally acts on the mold, there is also concern that the mold may be deformed, or the mold itself may be damaged. In FIG. 9B, Reference numeral 2 denotes a fixed platen, and reference numeral 6 denotes tie bars 6.
In order to reduce a difference in pressure depending on locations of a platen, it is effective to improve rigidity of the platen, but in a method of simply increasing a thickness of the platen, the platen weight increases, which leads to increased costs. For that reason, a shape capable of improving rigidity without increasing the weight compared to the related art has been proposed.
Japanese Utility Model Application Laid-Open No. 7-35020 discloses a technique in which, in at least one of a fixed platen and a movable platen, a notch portion reaching a tie bar insertion hole from an outer circumferential portion for preventing the deformation of a mold mounting surface is provided at least in a region of the tie bar insertion hole in an intermediate portion in its thickness direction of the platen.
However, in the above-described technique, in case where a notch is provided over the entire circumference by increasing a notch portion, in some cases, an outer circumferential portion of mold mounting portion cannot fully support the mold clamping force, as a result, the outer circumferential portion of a mold mounting portion may be deformed in the opposite direction such that the outer circumferential portion of the mold may open. In addition, in some cases, due to an increase in deformation of a tie bar fixing portion, the tie bar is bended, and thus, failures such as break of the tie bar may occur.
WO 2005/084909 A discloses a technique in which, as illustrated in FIG. 10A (front view) and FIG. 10B (side view), a central region of a mold mounting portion and a central region of a back portion of a mold support device are connected by a central connecting member, and an outer circumferential portion of the mold mounting portion and an outer circumferential portion of the back portion of the mold support device are connected by an outer circumferential portion connecting member, with the result that a portion for transmitting a load to the mold mounting portion of the fixed platen 2 is provided in the central portion and in the outer circumferential portion so as not to open the outer circumferential portion.
However, in the above-described technique, since the portion for transmitting a load to the mold mounting portion of the fixed platen 2 is divided into the central portion and the outer circumferential portion with a space left therebetween, as a result, depending on the size of the mold, the outer circumferential portion of the mold may be located in the space so that a force is not transmitted to the outer circumferential portion of the mold. Furthermore, when a large mold is mounted, as illustrated in FIG. 10B, its outer circumferential portion may be deformed toward a cored opening, and therefore, a gap may be generated on the outer circumferential portion of the mold and burrs may occur.
Japanese Patent Application Laid-Open No. 2010-111018 discloses a technique in which, to cope with concentration of the pressure on the mold corners due to deformation of the fixed platen, a portion for transmitting a load to the mold mounting portion of the movable platen is provided in a central portion and an extension portion vertically extending from the central portion so that an intermediate pressure between the corners is increased, thereby reducing a pressure difference generated along a joint surface between the fixed mold and the movable mold so as to be uniform.
However, in the above-described technique, the load transmitting portion having the central portion and the extension portion vertically extending from the central portion is provided in the movable platen so that the uniform pressure distribution of the mold is achieved by matching the deformation of the mold mounting surface of the movable platen with the deformation of the mold mounting surface of the fixed platen. However, since the extension portion is provided only in the vertical direction, in some cases, the adjustment of other directions cannot be performed well.